There has been considerable interest in light emitting organic materials such as conjugated polymers for a number of years. Light emitting polymers possess a delocalised pi-electron system along the polymer backbone. The delocalised pi-electron system confers semiconducting properties to the polymer and gives it the ability to support positive and negative charge carriers with high mobilities along the polymer chain.
Thin films of these conjugated polymers can be used in the preparation of optical devices such as light-emitting devices. These devices have numerous advantages over devices prepared using conventional semiconducting materials, including the possibility of wide area displays, low DC working voltages and simplicity of manufacture. Devices of this type are described in, for example, WO-A-90/13148, U.S. Pat. No. 5,512,654 and WO-A-95/06400.
Great efforts have been dedicated to the realization of a full-colour, all plastic screen. The major challenges to achieve this goal are: (1) access to conjugated polymers emitting light of the three basic colours red, green and blue; and (2) the conjugated polymers must be easy to process and fabricate into full-colour display structures. OLEDs are effective in meeting the first requirement, since manipulation of the emission colour can be achieved by changing the chemical structure of the organic emissive compound.
However, while modulation of the chemical nature of the emissive layer is often relatively easy and inexpensive on the lab scale it can be an expensive and complicated process on the industrial scale. The second requirement of the easy processability and build-up of full colour matrix devices raises the question of how to micro-pattern fine multicolour pixels and how to achieve full-colour emission. Inkjet printing, hybrid inkjet printing technology and spin coating are examples of suitable technologies that can be adopted to apply the polymer solutions in the desired pattern.
At their most basic, organic electroluminescent devices generally comprise an organic light emitting material which is positioned between a hole injecting electrode and an electron injecting electrode. The hole injecting electrode (anode) is typically a transparent tin-doped indium oxide (ITO)-coated glass substrate. The material commonly used for the electron injecting electrode (cathode) is a low work function metal such as calcium or aluminium.
The materials that are commonly used for the organic light emitting layer include conjugated polymers such as poly-phenylene-vinylene (PPV) and derivatives thereof (see, for example, WO-A-90/13148), polyfluorene derivatives (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629, WO-A-00/55927 and Bernius et al., Adv. Materials, 2000, 12, No. 23, 1737), polynaphthylene derivatives and polyphenanthrenyl derivatives; and small organic molecules such as aluminium quinolinol complexes (Alq3 complexes: see, for example U.S. Pat. No. 4,539,507) and quinacridone, rubrene and styryl dyes (see, for example, JP-A-264692/1988). The organic light emitting layer can comprise mixtures or discrete layers of two or more different emissive organic materials.
Typical device architectures are disclosed in, for example, WO-A-90/13148; U.S. Pat. No. 5,512,654; WO-A-95/06400; R. F. Service, Science 1998, 279, 1135; Wudl et al., Appl. Phys. Lett. 1998, 73, 2561; J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660; T. R. Hebner, C. C. Wu, D. Marcy, M. L. Lu, J. Sturm, Appl. Phys. Lett. 1998, 72, 519); and WO 99/48160.
The injection of holes from the hole injecting layer such as ITO into the organic emissive layer is controlled by the energy difference between the hole injecting layer work function and the highest occupied molecular orbital (HOMO) of the emissive material, and the chemical interaction at the interface between the hole injecting layer and the emissive material. The deposition of high work function organic materials on the hole injecting layer, such as poly (styrene sulfonate)-doped poly (3,4-ethylene dioxythiophene) (PEDOT/PSS), N,N′-diphenyl-N, N′-(2-naphthyl)-(1, 1′-phenyl)-4, 4′-diamine (NBP) and N,N′-bis(3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD), provides hole transport layers (HTLs) which facilitate the hole injection into the light emitting layer, transport holes stably from the hole injecting electrode and obstruct electrons. These layers are effective in increasing the number of holes introduced into the light emitting layer. However, the surface of ITO is not well defined and the chemistry at the interface with these conventional hole transport materials is hard to control.
As an alternative to the high work function organic materials such as PEDOT/PSS, high resistivity inorganic layers have been proposed for use as hole transport layers in, for example, EP-A-1009045, EP-A-1022789, EP-A-1030539 and EP-A-1041654. EP-A-1022789 discloses an inorganic hole transport layer which is capable of blocking electrons and has conduction paths for holes. The layer has a high resistivity, stated to be preferably in the region of 103 to 108 Ω-cm. The materials which are disclosed have the general formula (Si1-xGex)Oy wherein 0≦x≦1 and 1.7≦y≦2.2. The work function of this hole transport layer is not well defined and is likely to vary depending upon the actual identity of x and y.
More recently, Chen et al, Applied Physics Letters 87, 241121 (2005) has disclosed a connecting structure for tandem organic light-emitting devices. The connecting structure consists of a thin metal layer as the common electrode, a hole-injection layer (HIL) containing molybdenum trioxide on one side of the common electrode, and an electron-injection layer involving Cs2CO3 on the other side. Such a connecting structure permits opposite hole and electron injection into two adjacent emitting units and gives tandem devices superior electrical and optical performances. The structure is prepared wholly by thermal evaporation.
Kanai et al, Organic Electronics 11, 188-194 (2010) discloses that an electronic structure at the α-NPD/MoO3/Au interfaces has been investigated (molybdenum trioxide deposied by thermal evaporation). It was found that the molybdenum trioxide layer contains a number of oxygen vacancies prior to any treatment and gap states are induced by the partial filling of the unoccupied 4d orbitals of molybdenum atoms neighbouring oxygen vacancies. The α-NPD thickness dependence of XPS spectra for the α-NPD/MoO3 system clearly showed that molybdenum atoms at the surface of the molybdenum trioxide film were reduced by α-NPD deposition through the charge-transfer interaction between the adsorbed α-NPD and the molybdenum atoms. This reduction at the α-NPD/MoO3 interface formed a large interface dipole layer. The deduced energy-level diagram for the α-NPD/MoO3/Au interfaces describes the energy-level matching that explains well the significant reduction in the hole-injection barrier due to the molybdenum trioxide buffer layer.
Bolink et al, Adv. Funct. Mater. 18, 145-150 (2008) discloses a form of bottom-emission electroluminescent device in which a metal oxide is used as the electron-injecting contact. The preparation of the device comprises thermal deposition of a thin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solution processing of the light-emitting layer and subsequently the deposition of a high-workfunction (air-stable) metal anode. The authors showed that the device only operated after the insertion of an additional hole-injection layer in between the light-emitting polymer (LEP) and the metal anode.
In summary, this recent prior art describes the use of thermally evaporated molybdenum trioxide as either hole injecting layers (HILs) or as electron injecting layers. However, while the use of molybdenum trioxide and potentially other transition metal oxides as a hole injecting layer to dope the interface between an anode and a semiconducting hole transport layer improves the efficiency of injection of holes from the hole injecting anode to the semiconducting layer, the thermal evaporation techniques used to deposit the molybdenum trioxide HILs are not ideal for scaling up for use on a manufacturing scale.
There is therefore a need for an improved process for the preparation of OLED devices that is solution-based, overcoming the scale-up problems noted above for the prior art thermal deposition techniques and which allows the tuning of the p-acceptor strength of the resulting deposited transition metal oxide layer acting as a hole injection layer to the HOMO levels (i.e. ionisation potentials) of the hole transporting conjugated materials.